Frequency Encode All?
Why not just frequency-encode all the voxels?
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Since frequency-encoding seems relatively simple and straightforward, why make things complicated by introducing phase-encoding, parallel imaging, and other confusing concepts? Why not just design gradients so that every voxel has a different frequency and frequency-encode everything?
This sounds like a great idea at first, but a few sketches will show you that it is impossible to construct a gradient field so that each pixel in a slice or volume has a unique frequency. No matter how such a gradient is designed, there will always be two or more voxels with the same frequency; therefore, a scheme based on assigning a unique frequency to every point in a subject simultaneously is not tenable.
It is possible, however, to oscillate the gradients so that over time each pixel is assigned a unique frequency or physically move the patient within the magnet from point to point. Both these strategies were used in the early days of MR imaging and were known as sensitive point techniques. Damadian's original field-focused nuclear magnetic resonance (FONAR) used such a method. However, sensitive point techniques are extremely slow and have largely been abandoned in modern imaging except for some limited spectroscopic applications.
It is possible, however, to oscillate the gradients so that over time each pixel is assigned a unique frequency or physically move the patient within the magnet from point to point. Both these strategies were used in the early days of MR imaging and were known as sensitive point techniques. Damadian's original field-focused nuclear magnetic resonance (FONAR) used such a method. However, sensitive point techniques are extremely slow and have largely been abandoned in modern imaging except for some limited spectroscopic applications.
References
Damadian R. Field focusing n.m.r (FONAR) and the formation of chemical images in man. Phil Trans R Soc Lond B 1980; 289:489-500
Hinshaw WS. Image formation by nuclear magnetic resonance: the sensitive-point method. J Appl Phys 1976; 47:3709-3721.
Lai CM, Lauterbur PC. True three-dimensional image reconstruction by nuclear magnetic resonance zeugmatography. Phys Med Bio 1981; 26:851-856.
Damadian R. Field focusing n.m.r (FONAR) and the formation of chemical images in man. Phil Trans R Soc Lond B 1980; 289:489-500
Hinshaw WS. Image formation by nuclear magnetic resonance: the sensitive-point method. J Appl Phys 1976; 47:3709-3721.
Lai CM, Lauterbur PC. True three-dimensional image reconstruction by nuclear magnetic resonance zeugmatography. Phys Med Bio 1981; 26:851-856.
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